A conventional successive-approximation ADC includes a digital-to-analog converter (DAC), a comparator, and a successive approximation register (SAR). The comparator receives the input voltage and the output of the DAC. The DAC receives the output of the SAR. On the first cycle of the conversion, the SAR begins by producing a digital output that is one-half the full-scale value. On the second cycle the SAR produces an output that is halfway between its previous output and zero if the comparator indicates that the input voltage is less than the output of the DAC, and produces an output that is halfway between its previous output and the full-scale value if the comparator indicates that the input voltage is greater than the output of the DAC. On successive cycles, the SAR continues to produce outputs halfway between two previous outputs or between a previous output and zero or between a previous output and the full-scale value. A successive approximation ADC thus performs a binary search until it arrives at a digital output that corresponds most closely to the analog input.
Several types of DACs are known in the art. The simplest type of DAC is known as a weighted-resistor DAC because it includes a network of resistors connected to a summing node. The bits of the input word are connected to corresponding switches, such as transistors. Each switch connects one of the resistors into the network if the corresponding bit is high ("1") and disconnects the resistor from the network if the corresponding bit is low ("0"). The resistor has a value that is weighted according to the position of the bit. The summing node sums the currents contributed by the resistors that are switched into the network, thereby producing a current that corresponds to the digital input. An op-amp is typically included to convert this current into a voltage output. A modified type of weighted-resistor DAC, known as a R-2R ladder DAC, has a resistor network that minimizes the range of resistor values. Similar types of DACs are known that use a capacitor network rather than a resistor network. A related type of DAC, known as a binary-weighted current sink DAC, includes weighted current sources rather than weighted resistors. Each bit controls a group of transistors, such as bipolar transistors or metal-oxide semiconductor field-effect transistors (MOSFETs), corresponding in number to the weight of the bit. The number of transistors in the group controlled by the n.sup.th bit is 2.sup.n. The gates of the MOSFETs in each group are coupled to each other and to the controlling bit. The sources and drains of the MOSFETs in each group are also coupled to each other. An N-bit binary-weighted current sink DAC thus requires 2.sup.n -1 transistors.
A DAC is typically implemented in an integrated circuit or chip. The precision of the resistors and the offset voltage of the transistors of a DAC affect its accuracy. Current sink DACs are preferred because they minimize the number of resistors and thus the cumulative error. Current sink DACs can be implemented using common chip fabrication processes such as the bipolar metal-oxide semiconductor (BiMOS), complementary metal-oxide semiconductor (CMOS), and bipolar complementary metal-oxide semiconductor (BiCMOS) process. CMOS and BiCMOS chips are advantageous because they are more power-efficient than many other processes.
ADCs are available as either differential or single-ended input devices. ADCs are also available as voltage mode devices or current mode devices. Depending on circuit design constraints a designer will use one of these four types of ADCs in their design. Because there are so many possible configurations needed, manufactures must design and maintain many different types of ADCs. This is costly in both the design and configuration control stages.
Furthermore, as mentioned above, conventional successive approximation ADCs use a large resistor chain which in turn results in a large overall circuit. As circuit designs become more compact, use of such large ADCs is a great disadvantage.
These problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.